Actually it is not uncommon or abnormal for grooves to appear in the surfaces of the rotors.
Your brakes are exposed to a lot of road debris and from time to time a tiny pebble, piece of metal, or other debris will get wedged between the rotor and the brake pad and dig a small groove in the rotor.
On my 2001 Sport Trac I hit the brakes coming off a freeway ramp and noticed that I had lost a lot of my braking power and heard a grinding noise. My brake power returned immediately, but I was very concerned. I looked at the rotors and saw a significate groove on the outside of the passenger side rotor. I assumed my pads were gone and immediately bought new Raybesto's QuietStop Carbon-Metalic pads.
When I took the calipers off I noticed that I had plenty of pad all the way around, but that a large piece of road debris had wedged between the pad and the rotor and was forced through the pad leaving a groove in the pad and the rotor. I replace the brake pads anyway since I had already bought them. I had 37K miles on the truck and was nearly at the end of my lease, and had planned on turning it back in and getting an new 2003 or 2004 model.
If you have not warn the pads down to the point where you are getting metal to metal contact with the rotor, grooves in the rotors are very normal.
...Rich